The recovery of elemental sulfur from hydrogen sulfide-containing gas streams is known in the prior art as disclosed in the article "Fundamentals of Sulfur Recovered by the Claus Process" by B. Gene Goar, published in the 1977 Gas Conditioning Conference Report.
In a series of four articles published in the Canadian Gas Journal, Gas Processing/Canada, July-August 1970, p. 38, September-October 1970, p. 32, January-February 1971, p. 12 and July-August 1971, p. 16, titled. "Computer Design and Simulation of Sulfur Plants", R. S. Lees and J. T. Ryan describe the kinetics typical of Caus processes with emphasis on reaction furnace operation and reverse reactions that occur upon cooling in a waste heat boiler.
Oxygen-enrichment in the operation of a Claus sulfur plant to increase the capacity of hydrogen sulfide treated in such a plant has also been disclosed in the article "Oxygen Use in Claus Sulfur Plants" by M. R. Gray and W. Y. Svrcek, published in the 1981 Gas Conditioning Conference Report. It was disclosed more specifically that oxygen can be added to the air feed to the burner of a reaction furnace in a Claus sulfur plant to increase the amount of hydrogen sulfide which is combusted to sulfur dioxide for later catalytic conversion to elemental liquid sulfur product. The maximum capacity increase which can be achieved with oxygen-enrichment is determined by the pressure drop through the plant, the reactor space velocity and temperatures of the reaction furnace and the various catalytic zones, particularly the refractory materials used in the furnace of the Claus plant.
In the 1983 publication by Linde of Union Carbide entitled "Claus Plant Oxygen Enrichment", it is noted that oxygen-enrichment limitations exist for rich hydrogen sulfide streams due to temperature limits in the furnace or waste heat boiler of the Claus plant.
U.S. Pat. No. 3,822,341 discloses a Claus plant which uses oxygen-enrichment. One source of the oxygen is initially used to strip residual SO.sub.2 from a sidestream in vessel 92, before the oxygen stream in line 96 is optionally recycled with the oxygen in line 12 going to the combustion zone of the waste heat boiler 8, as recited at col. 5, lines 65-68 of the specification. Because the oxygen content of such a stream is completely consumed in the exothermc reaction, this stream cannot be utilized as a moderating medium for flame temperature of the reaction furnace. As described by the Goar article above, Claus sulfur plants typically have an adiabatic reaction furnace followed by a waste heat boiler. The excessive temperature problem with oxygen-enriched operation occurs in the adiabatic reaction furnace. U.S. Pat. No. 3,822,341 ignores the existence of this problem.
U.S. Pat. No. 4,153,674 discloses a Claus plant and tail gas clean up plant wherein a gas stream in line 20 is removed from a tail gas system and is returned or recycled to the front end of the Claus plant 7. This patent does not consider oxygen-enrichment or flame temperature moderation by a recycle stream. Also, a tail gas is reacted to convert all sulfur to hydrogen sulfide, which is absorbed, stripped and returned to the Claus plant.
U.S. Pat. No. 4,279,882 discloses a sulfur recovery process which uses only a series of catalytic reaction beds rather than a combustion reaction furnace, as in the traditional Claus plant. A temperature modifying recycle stream is set forth in the patent, wherein stream 26 is returned to the feed in order to control the temperature in the catalytic reaction zones. This process is economical only for dilute hydrogen sulfide feed gas applications. It also requires a recycle blower operating at high temperature.
It is also known to recycle liquid sulfur product from a Claus plant to the reaction furnace of a Claus plant when processing dilute feed gas streams to such a Claus plant. In an article by H. Grekel, J. W. Palm and J. W. Kilmer in the Oil and Gas Journal. Oct. 28, 1968, page 88+, a scheme is set forth in FIG. 1 of the article to process a feed of 2-15% hydrogen sulfide. Below 15 vol %, Claus reaction furnace flame temperatures are too low for stable operation. Grekel, et al. burn one third of the Claus product sulfur to provide additional sulfur dioxide, such burning taking place in the reaction furnace with air. Some feed is also introduced to the furnace to moderate the temperatures of combustion. The resulting sulfur dioxide is used for the conversion of the predominant amount of the dilute feed entering the catalytic reaction zone wherein the net sulfur product is produced. This process approach is limited to dilute hydrogen sulfide feed applications. Oxygen use is not involved.
In the article "Sulfur From Hydrogen Sulfide" by B. W. Gamson and R. H. Elkins in Chemical Engineering Progress, Vol. 49, No. 4, at page 203 (1953) a Claus process is disclosed. In FIG. 18, the recycle of sulfur from a liquid sulfur pit of a Claus plant is indicated wherein the sulfur is returned to a sulfur burner in order to produce sulfur dioxide using air. The sulfur dioxide is cooled prior to being mixed with a dilute hydrogen sulfide stream prior to catalytic conversion of the hydrogen sulfide and sulfur dioxide to liquid sulfur. Again, this disclosure recycles sulfur and burns it to sulfur dioxide to process a dilute acid gas feed stream and not to avoid excessively high temperatures. There is no disclosure of using oxygen enrichment with such a recycle.
U.S. Pat. No. 4,302,434 discloses a process for producing hydrogen and sulfur from a hydrogen sulfide feed wherein the hydrogen sulfide feed is predominantly cracked at high temperatures rather than combusted with an oxidant gas. At least some hydrogen sulfide can be burned in addition to that being cracked. The cracked hydrogen sulfide components of hydrogen and sulfur are quickly cooled by indirect heat exchange to below 1500.degree. F. in a waste heat boiler in order to avoid the recombination of the cracked components to hydrogen sulfide. After condensing and removing liquid sulfur, residual sulfur compounds are rehydrogenated to hydrogen sulfide for solvent removal to provide a hydrogen rich final product process. Essentially no sulfur dioxide is produced in the process and no catalytic Claus conversion steps requiring a 2:1 H.sub.2 S:SO.sub.2 ratio for efficient conversion to sulfur are involved.
In U.S. Pat. No. 4,481,181 a process is set forth for production of hydrogen from hydrogen sulfide wherein hydrogen sulfide is combusted with oxygen to accomplish partial oxidation of the hydrogen sulfide with less than stoichiometric quantities of oxygen and the partial oxidation product is quenched with a cooler recycle gas stream to prevent recombination of hydrogen and sulfur. The feed hydrogen sulfide is preheated to the maximum temperature practical (1150.degree. K.) to obtain the high temperature (1400.degree. K.) necessary for substantial endothermic, equilibrium cracking of hydrogen sulfide to hydrogen and sulfur vapor. while minimizing the input oxygen required to combust part of the hydrogen formed to provide the heat required for the endothermic cracking reaction. Under these conditions, as set forth in the example, the overall reactions to equilibrium are: EQU 83H.sub.2 S+6O.sub.2 .fwdarw.51H.sub.2 S+20H.sub.2 +16S.sub.2 +12H.sub.2 O
At these conditions, substantially no sulfur dioxide is produced and the effluent gas does not have the 2:1 H.sub.2 S:SO.sub.2 mole ratio necessary for efficient conversion to sulfur in the typical catalytic Claus conversion process steps. Hydrogen is recovered from the quench cooled effluent gases after removal of a recycle stream to perform the quench.
The present invention overcomes the shortcomings of the prior art by increasing throughput of a Claus plant with oxygen-enrichment to an extent beyond that considered feasible in the prior art because of flame temperature limitations. In addition. the present invention provides better throughput of reaction components through the Claus plant reaction train by reducing the carryover of inerts through the system. This is achieved by injecting elemental sulfur or recycling elemental sulfur into the reaction furnace of the Claus plant from the product sulfur produced in the overall Claus plant. The sulfur injection decreases pressure drop in the downstream portion of the Claus plant which pressure drop would have been increased with other injectants, such as water.